An example of an electromagnetically operated control valve is shown in U.S. Pat. No. 4,538,645 issued to Perach entitled "Control Valve Assembly" the assignee of which is a wholly owned subsidiary of the assignee of this invention. The control valve assembly includes a pair of valve seats and a poppet which is oscillated between the seats.
An electromagnet is provided to drive the poppet between the seats. The electromagnet includes a solenoid assembly and a spring loaded armature of magnetic material. The solenoid assembly has a non-magnetic form, commonly called a bobbin, in which the armature moves and a solenoid or coil of insulated wire wound around the bobbin and about the armature. An iron or steel casing, such as the shell 40, usually extends circumferentially about the solenoid. The casing increases the mechanical force of the plunger and concentrates the lines of flux of the magnetic field which is generated by energizing the coil.
Another example of a solenoid assembly is similar to the solenoid assembly shown in FIG. 1 of this application. This solenoid assembly has an outer steel casing and an inner plastic casing. An injection molded thermoplastic is used as an encapsulant to form the inner casing about the solenoid. The encapsulant is commonly either the same plastic as the bobbin or sometimes a dissimilar plastic.
The encapsulant overlaps either end of the flanges of the bobbin to provide a seal from the outside environment. As shown, the adjacent material or the flange might be recessed to provide for this overlap. The overlapped encapsulant provides a reasonable seal from the outside environment but not one that permits submergence of the solenoid assembly in a liquid or washing of the solenoid assembly with a high pressure stream of liquid as might occur during normal cleaning operations of a machine on which the solenoid is installed. Experience has also shown that cyclic variations in temperature tend to open a gap between the flanges of the bobbin and the encapsulant further decreasing the isolation of the interior of the coil from the outside environment.
Sealing the coil from the outside environment is important because contamination of the coil with water or other electrically conductive substances may degrade the performance of the coil by breaking down the insulation of the wire and even cause shorting of the coil to ground through the shell.
As a result, scientists and engineers are seeking to develop a solenoid assembly which can accept submersion of the coil in a liquid for a short period of time or the impact of a high pressure liquid stream without allowing the leakage of the fluid through the casing into the coil.